Effect of river straightening on the hydrochemical properties of floodplain lakes: Observations from the Lyna and Drweca Rivers, N Poland

The aim of the paper is to present two independent approaches to the issue of preservation of former meander loops based on examples of hydrotechnical works undertaken along meandering sections of the Drweca and Lyna Rivers (N Poland) in the 20th century. Except for significant changes in morphology of both rivers (shortened lengths, decrease in sinuosities and increase in channel slopes), the direct effect of their straightening is numerous cut-off channels. Hydrotechnical works performed along the middle section of the Lyna River maintained the connections between the active river channel and downstream arms of former meander loops. Thanks to that connection, the new water bodies successfully function as biogeochemical filters in the river valley and create favorable conditions for biodiversity. The consequence of the channelization of the Drweca River is numerous cut-offs separated from the river channel and characterized by advanced processes of eutrophication. The water quality data collected within the period of 2004-2006 showed significant consequences of the performed works for the man-made ecosystems. The comparison of water quality in semi-lotic and lentic cut-offs in relation to water in their parent rivers showed statistical differences. Significantly high concentrations of orthophosphates, total phosphorus and sulphates as well as the lowest concentrations of nitrates characterized lentic environments along the Drweca River. Besides hydrological connectivity, an important factor responsible for transformation processes of the man-made ecosystems is hydrological regime. It influences ecological integrity in flood-plain ecosystems during high water levels and largely controls their chemical characteristics and aeration. Flood disturbances for lentic water bodies may renew connectivity and reset succession, can hold up the trend towards terrestrialization and disappearance of man-made water bodies in the floodplain. However, in light of the conducted studies, the maintenance of the connection between the downstream arm of the cut-off channel and the river is highly recommended. (C) 2010 Elsevier B.V. All rights reserved.